The invention relates to a temperature control method particularly in appliances having a heatable compartment such as heating cabinets, incubators, sterilizers or thermal baths.
The compartment is heated by a heating element supplied with heating power by line voltage regulated by a controllable switch which can be turned on at a controllable phase angle of the line voltage. A deviation voltage is obtained by forming the difference between a measuring voltage representing an actual temperature in the compartment and a desire voltage representing a desired temperature in the compartment. The phase angle of the controllable switch is shifted in proportion to the deviation voltage. A line-synchronous delta voltage having a waveform presenting a descending edge region is generated from the line voltage. The deviation voltage is compared with the descending edge region of the line-synchronous delta voltage during each half wave of the line voltage and an output signal is generated which controls the position of the phase angle for turning on the controllable switch from the time when the amplitudes of the desired voltage and the actual voltage have the same amplitude.
A known temperature control method in a limited space (control path) which is often employed because of its simple circuitry realization possibilities is the so-called two-point method in which the correcting variable is fed to the control circuit in the form of an 0-1 function. In this method, heating continues at full power in a temperature control circuit until the desired temperature is reached. Then the heating element is disconnected. The remaining residual heat of the heating system, however, heats the control path to above the desired temperature, i.e. the correcting variable overshoots. When the temperature falls below the desired temperature, the full heating power is turned on again until the desired temperature is again attained. Thus, the actual, two-point regulated temperature fluctuates over a relatively broad range around the desired temperature.
If, for application specific reasons, the desired temperature is to be regulated more precisely, more expensive regulators must be employed, for example proportional or proportional/integral differential regulators.
In an electronic proportional regulator (P regulator) for temperature regulation, a measured value detection component generates a measuring voltage which is representative of the actual temperature and an adjustment member generates a desired voltage which is representative of the desired temperature. From the measuring voltage and the desired voltage, a difference former forms a deviation voltage which corresponds to the deviation between the desired temperature and the actual temperature. The latter serves as a measure for the heating power to be fed into the system to be regulated. Thus, the heating power is proportional to the deviation. In particular, if a system temperature is regulated which lies at a temperature level above room temperature, an essentially constant component, which is a function of the absolute amount of the desired temperature, must be included in the percentage of the heating power which is proportional to the deviation. Essentially, this is the so-called integral component in a proportional/integral regulator.
In heating systems supplied with line voltage, the heating power is generally regulated in proportion with the deviation by way of phase angle control. This means that the phase angle is set to correspond to the deviation.